1. Field of the Invention
The invention is related to the fields of aviation and sensors, and in particular, to sensor systems for aerial vehicles.
2. Description of the Prior Art
Aircraft have typically used two fundamental types of air samplers. The first type is called a total air temperature (TAT) probe that obtains total (dynamic) air temperature and static (ambient) air temperature. This TAT probe extends from the aircraft skin about 3 inches, which is away from the friction-heated boundary layer of air next to the aircraft's metal surface. The TAT probe measures the dynamic (total) temperature and obtains the static temperature through the equation:TT=TS(1+0.2M2)
where TT is the total temperature;                TS is the static temperature; and        M is the Mach number which is the fractional speed of the aircraft relative to the speed of sound.        
The TAT probe includes a probe heater, which is an FAA requirement due to icing concerns. One problem is the heater tends to fail, which is the highest failure mode of the probes.
The second type of probe is called a pitot tube and is used to measure differential pressure (total minus static) for subsequent calculation of aircraft velocity through Bernoulli's equation:V2=2(PT−PS)/ρ
where V is velocity;                PT is total pressure;        PS is static pressure; and        ρ is the density of air, which is a function of atmospheric pressure and temperature.        
These two probes work together to provide the information needed for efficient flight. Both types of probes have the common feature of extending away from the airframe to avoid contaminated measurements induced by boundary layer effects near the aircraft's skin. One problem with these two probes is the frictional drag from the extension of both probe from the aircraft's skin. The TAT probe has a frictional drag that is an effective 2.5 Ibs. Over time, the cost of additional fuel for such additional weight ranges from $1–$2 per pound per week per aircraft. Another problem arises when the probes are applied to stealth aircrafts. Both of the probes increase the radar cross section, which increases the radar visibility of the aircraft.
Another important measurement for aircraft is water vapor. Water vapor affects virtually all aspects of aviation weather and thus, the safety, efficiency, and capacity of an airspace system. For example, summertime convection is behind most traffic delays. Weather prediction in general, but especially precipitation and severe storm prediction, are crucially dependent upon accurate water vapor profiles in the lower troposphere. The commercial aircraft real-time ascent and descent reports can provide profiles of winds, temperature, and water vapor.
One prior system has used the TAT probe in combination with a water vapor sensing system. FIG. 1 depicts a prior system with the TAT probe and a water vapor sensing system in the prior art. The prior system includes a standard TAT probe to measure total air temperature and static air temperature from the air flow. The water vapor sensing system includes a diode laser to measure the water vapor. This prior system was tested in a prototype mode but never built as a commercial product because of the limited space available within the TAT probe. This forced the use of fiber optic cables to carry the laser light and these induced optical “fringes” that reduced sensitivity of the diode laser measurement technique.
Another prior system uses an “open path” for diode lasers to measure water vapor. The laser transmitter and receiver are external to the aircraft. However, this prior system has accuracy and solar interference problems in addition to the drag concerns.